1. Field of the Invention
The present invention relates to a plastic optical fiber which transmits visible ray with a small transmission loss and a process for producing the same.
2. Description of the Prior Art
As to optical fibers, those of inorganic glass having superior light transmitting properties over a wide range of wavelengths have so far been known. However, inorganic glass type optical fibers are inferior in processability and in resistance to flexural stress, and they are expensive. Accordingly, optical fibers mainly composed of synthetic resins have been developed. Plastic optical fibers are composed, for example, of as a core material, a polymer having a higher refractive index and superior light transmitting properties and as a cladding material, a transparent polymer having a lower refractive index to shape a core-and-cladding structure. As the core material polymer having superior light transmitting properties, amorphous polymers are preferred and polymethyl methacrylate, polycarbonate or polystyrene has generally been used.
Among these polymers, polymethyl methacrylate is superior not only in light transmitting properties but also in mechanical properties, weather resistance, etc., and therefore, it has been used as a core material for high-performance, plastic optical fibers on a commercial scale. However, plastic optical fibers using polymethyl methacrylate as the core material has a glass transition temperature (Tg) of 100.degree. C. so that it cannot be used at all when its service environmental temperature exceeds 100.degree. C. Thus, such a limit of the service temperature of polymethyl methacrylate has restricted its use for plastic optical fibers.
On the other hand, as to plastic optical fibers using polycarbonate as the core material, various proposals have been made. However, as compared with fibers using polymethyl methacrylate, as the core material, fibers using polycarbonate are far inferior particularly in light transmitting properties as to light of visible light wavelengths. Hence presently they have not yet been commercially employed for practical use in spite of their superior heat resistance.
For example, the transmitting characteristics of an optical fiber using polycarbonate as the core material, reported by Tanaka et al in Japanese Electronic Correspondence Society, Collective, Wh Country Grand Meeting, 1985 (collected previous manuscripts) is shown in FIG. 10 of the accompanying drawings. Namely, at the wavelengths of 775 nm, 820 nm and 948 nm, the loss is minimum (5,100 dB/km), whereas within a wavelength region of blue color to green color, light of 500 nm is not transmitted at all. Further, as to an optical fiber using polycarbonate as the core material, too, disclosed in Japanese patent application laid-open No. 57-46204/1982, light transmission has been observed only within an infrared ray region of 950 nm wavelength.
The present inventors have made extensive research in order to overcome the drawback of conventional optical fibers having polycarbonate as the core material, and as a result have found an optical fiber having a little variation in the light transmitting characteristics within a visible light region.